An example of electromagnetic valve used in a fuel injection valve of a common rail system is disclosed in Patent Literature 1. The fuel injection valve disclosed in Patent Literature 1 is composed of components including a nozzle body 2, a needle 3, a holder body 4, an orifice plate 6, and an electromagnetic unit 8, as shown in FIG. 4. The nozzle body 2 is coupled by a retaining nut 10 to the lower end portion of the body holder 4 with the orifice plate 6 disposed therebetween. A guide hole 12 is formed in the nozzle body 2 to extend therethrough from the top end surface to the tip end of the nozzle body 2. The needle 3 is disposed in the guide hole 12 in such a manner that it can freely slide in the guide hole 12. An injection port 14 is formed in the tip end of the guide hole 12 through which fuel is injected when the needle 3 moves upward. A high-pressure path 16 is formed by a gap between the inner circumferential surface of the guide hole 12 and the outer circumferential surface of the needle 3. The high-pressure path 16 guides highly pressurized fuel to the injection port 14. At a location between the two ends of the guide hole 12, a fuel reservoir 18 is formed by enlarging the inner diameter of the guide hole 12. The upper end of the high-pressure path 16 opens in the top end surface of the nozzle body 2 and is connected to a high-pressure path 20 in the orifice plate 6. The high-pressure path 20 is connected via a high-pressure path 22 in the holder body 4 to a pipe joint 24 disposed at the upper end of the holder body 4, and the pipe joint 24 is supplied with high-pressure fuel from a common rail.
A cylindrical spring seat 26 is press-fitted into and secured to the guide hole 12, and a spring 28 is disposed between the spring seat 26 and the needle 3. The spring 28 urges the needle 3 in the direction to close the valve, or in the downward direction in FIG. 4. The inner circumferential surface of the spring seat 26 provides a back-pressure chamber 30 for providing a pressure of high-pressure fuel to the upper end surface of the needle 3 as a back pressure. This back pressure also urges the needle 3 in the valve-closing direction. The pressure of the high-pressure fuel in the fuel reservoir 18 urges the needle 3 in the direction to open the valve, or in the upward direction in FIG. 4.
As shown in FIG. 5, an inlet path 32 and an outlet path 34 are formed in the orifice plate 6. The inlet path 32 is a path through which high-pressure fuel flows from the high-pressure path 20 into the back-pressure chamber 30, and an outlet path 34 is a path through which the high-pressure fuel flows from the back-pressure chamber 30 to a low pressure side.
The electromagnetic unit 8 is housed in the holder body 4. The electromagnetic unit 8 includes a stator 38 having an electromagnetic coil 36 wound around a plastic bobbin. The electromagnetic unit 8 includes also an armature 40 facing and movable relative to the stator 38. The electromagnetic unit 8 further includes a ball valve 42 movable with the armature 40 to open and close the outlet path 34. The stator 38 has, in its center, a vertically extending spring housing hole 44 in which a spring 46 is housed. The spring 46 presses the armature 40 so that the ball valve 42 is pressed toward the outlet path 34. The lower portion of the stator 38 functions as a valve chest in which the ball valve 42 is housed and which is filled with low-pressure fuel flowing from the outlet path 34. An annular groove 48 is formed in the upper surface of the orifice plate 6. A straight groove 50 extends outward from the annular groove 48, and the low-pressure fuel in the valve chest flows out into a low-pressure path 52 through the groove 50.
The armature 40 has a disc member 54, which is disposed to face the stator 38 and form a magnetic circuit with the stator 38. A pedestal 56 is formed in the center of the disc member 54, and an abutting portion 58 extends from the pedestal 56 toward the ball valve 42. The ball valve 42 sits in the abutting portion 58. A plurality of through-holes 60 are concentrically formed in the disc member 54. Guide pins 62 are inserted into some of the through-holes 60. The guide pins 62 are secured to the orifice plate 6. The through-holes 60 are formed at such locations as to interrupt the magnetic circuit formed by the disc member 54 and the stator 38.
In a state where no electric power is being supplied to the electromagnetic coil 36, the ball valve 42 closes the outlet path 34, and, therefore, the hydraulic pressure in the back-pressure chamber 30 plus the force given by the spring 28 to urge the needle 3 in the direction to close the valve is larger than the hydraulic pressure in the fuel reservoir 18 which acts to urge the needle 3 in the direction to open the valve. Accordingly, the needle 3 closes the injection port 14 so that the fuel is not injected. When electric power is supplied to the electromagnetic coil 36, magnetic flux is generated around the electromagnetic coil 36, so that the stator 38 and the armature 40 are magnetized, causing the armature 40 to be attracted toward the stator 38. As a result, the armature 40 moves toward the stator 38 against the force of the spring 46, being guided by the guide pins 62. This causes the ball valve 42, receiving the hydraulic pressure in the back-pressure chamber 30, to open the outlet path 34 whereby the high-pressure fuel in the back-pressure chamber 30 is released into the valve chest of the ball valve 42. Then, the hydraulic pressure in the back-pressure chamber 30 decreases so that the force to move the needle 3 in the direction to open the valve becomes greater. This makes the needle 3 move upward so that the fuel is injected through the injection port 14.